1. Field of the Invention
The invention concerns membranes employed in X-ray and corpuscular projection lithography, e.g. as part of a stencil mask for ion-beam lithography.
2. Background of the Invention
In the manufacturing of e.g., semiconductors, X-ray and corpuscular beam lithography is used. Corpuscular beams include electrons, ions, but also neutral atoms and ionic or neutral molecules, e.g. hydrogen ions, H.sub.3.sup.+. In projection lithography membranes of various materials for masks to pattern the beam are used. The materials of these membranes can be of any material that can be formed as a thin layer and structured, including semiconductors, e.g. single-crystal .alpha.-silicon or polycrystalline silicon, metals, e.g. nickel, and insulators, e.g. silicon dioxide or aluminum oxide, many other materials can be, and are, used in the art. The life time of these membranes is limited by the effects of damage incurred to the membranes during irradiation. As for ion-beam lithography, a bare silicon membrane, which can be patterned and used as a stencil mask, can only withstand a total charge density of 0.2 mC/cm.sup.2 of irradiated ions before its intrinsic stress changes drastically. To meet the needs of future lithography in VLSI and ULSI circuits, a mask will have to be capable of withstanding up to ten million exposures. This is approximately the number of exposures that can take place between design generations. The ion dose required to fully expose photoresist is approximately 5.times.10.sup.12 ions/cm.sup.2. Therefore a stencil mask used in a proximity printer would be bombarded with 5.times.10.sup.19 ions/cm.sup.2 or 8 C/cm.sup.2 after ten million exposures. The ion beam could also be magnified or de-magnified as with an ion projection lithography system. With a demagnification of 4.times., the stencil mask would be bombarded with 3.125.times.10.sup.18 ions/cm.sup.2, which equates with a total charge density of 500 mC/cm.sup.2 at the stencil mask.
It has been shown previously that silicon membranes swell during lithium ion bombardment due to ion implantation into the silicon crystal interstitial positions. Hydrogen or helium ions bombarding a silicon membrane also cause swelling which, in this case, is the main cause for stress change in the membrane. Hydrogen diffuses out of the silicon membrane when the temperature is set to 450.degree. C. for approximately 30 minutes, while helium diffuses out of the silicon membrane at a temperature of 700.degree. C. for approximately 8 hours. After the thermal treatment, the silicon membrane returns to its original tension for small doses, e.g. about 0.2 mC/cm.sup.2 ; for higher doses, the membrane is permanently damaged.
With masks of a material different from silicon, the problem stays in principle unchanged, although the extent of stress change due to irradiation can vary and even reverse, e.g. silicon dioxide shows a compaction rather than swelling upon irradiation with hydrogen, helium or argon ions. It is obvious that this phenomenon is not limited to the irradiation with ions but also with electrically neutral atoms or molecules. Moreover, since the lattice is affected by not only the implantation of atoms or molecules, but also the impact of the energetic radiation itself, stress change effects will prevail also for irradiation with electrons or high-energy electromagnetic radiation, as e.g. X-rays. The following discussion mainly refers to ion projection lithography, but it is understood that the considerations presented in the following also apply, with only minor adaptions, as e.g. taking the respective equivalent doses of irradiation, for the more general case of X-ray and corpuscular projection lithography.
In order to increase the life of a pattern mask used for ion-beam lithography, it is necessary to prevent the mask from swelling/compaction. This is usually done by means of protective coatings, as described in the Bohlen et al. U.S. Pat. No. 4,448,865, an ideal ion-absorbing coating is characterized by the following properties:
a) the stress of the ion-absorbing coating should be in the order of or less than that of the silicon membrane; PA1 b) the stress of the ion-absorbing coating should not change more than 10% when implanted with hydrogen or helium ion doses exceeding 1 C/cm.sup.2, PA1 c) the stress of the ion-absorbing coating should not change more than 10% when stored under typical clean-room conditions for periods up to 1 year; PA1 d) the deposition and patterning of the ion-absorbing coating should be compatible with the mask fabrication sequence.
Especially the last requirement eliminates many potential candidates for the ion-absorbing layer. Carbon is a candidate film for this application since it can be easily patterned in oxygen; as additional advantages, it has only gaseous oxides and a high emissivity of between 0.7 and 0.8.
One technique to apply low-stress carbon films is sputtering. Another possible technique employs electron beam evaporation. The normal mode of sputtering is to place the substrate directly above the sputtering target. This so-called "on-axis" sputtering has been used extensively for the deposition of carbon films. However, carbon films produced by "on-axis" sputtering are diamond-like, have very high compressive stress, and should, when bombarded with ions, exhibit the problems discussed above, i.e., trapping of ions leading to swelling of the membrane, as well. Since implanted gasses cannot diffuse out of these films at room temperature, they are not suitable as ion-absorbing layers. The compressive stress of a diamond-like carbon film would also severely distort and/or destroy a membrane. The thin silicon membranes from which lithography masks are made have inherently about 10 MPa of tensile stress, whereas the intrinsic stress of diamond-like carbon is compressive and between 1 GPa and 14 GPa. In the conventional sputtering configuration, negative ions generated at the cathode bombard the substrate and create highstress, diamond-like material. Carbon has been used as a conductive layer and to prevent sputtering, but carbon has not been applied for protecting e.g. silicon from ion implantation. Recently, however, a new sputter deposition technique, known as "off-axis" sputtering, has been developed for depositing high temperature superconducting films. This technique uses a different geometry in that the deposition surfaces are positioned in the off-axis configuration relative to the sputter targets. The technique of "off-axis" sputtering has been used in the deposition of superconducting films, as described by Eom et al., Applied Physics Letters, vol. 55 (1989) p. 595.
It is an object of the present invention to provide a method to apply a carbon layer to membranes whose stress is small in comparison to the inner stress of the original membrane. It is another object of the present invention to provide a treatment to a carbon layer such that upon subsequent exposure, its stress does not change more than 10% with irradiation doses exceeding the equivalent of 500 mC/cm.sup.2. The present invention further aims at a carbon layer to membranes, e.g. silicon membranes, used in ion-beam lithography in which the implanted gasses can diffuse out of the coating layer before swelling of the layer occurs.